1. Field of the Invention
The present invention relates generally to electrical machine drive controls, and specifically, to a method and system for mitigating fault modes in electrical machine drives for hybrid electric vehicles, electric vehicles and other systems.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Also, hybrid electric vehicles (HEV), which combine a smaller ICE with electric motors into one vehicle, attempt to address these needs.
Understanding electrical drive operation is critical in the design of electric vehicles and hybrid electric vehicles. In particular, where permanent magnet electric motors are employed, diagnosis and mitigation of fault modes are critical. This is due in part to the continuous existence of permanent magnet flux, which may produce pulsation torque, over voltage or over current conditions during certain fault modes.
An electrical machine drive system includes a DC power source, an inverter, and an electrical machine. The DC power source is coupled to, and provides power to, the inverter. The electrical machine is coupled to the inverter, and is driven by the inverter. The electrical machine typically includes a first phase, a second phase and a third phase. The inverter to drive this electric machine includes a first stage, a second stage, and a third stage. The first stage of the inverter is coupled to the first phase of the electrical machine; the second stage of the inverter is coupled to the second phase of the electrical machine; and the third stage of the inverter is coupled to the third phase of the electrical machine. Each stage of the inverter has an upper power switch and a lower power switch. An upper diode is coupled across the upper power switch and a lower diode is coupled across the lower power switch.
Certain faults in the phases or stages of the electrical machine or inverter may result in undesirable characteristics. The faults may include open power switches, open diodes, shorted power switches, or shorted diodes, which may degrade system performance. For example, asymmetric operation of the electrical machine drive system due to an open or short may cause torque ripples, which may result in an undesirable ride, in the case of an electric or hybrid electric vehicle.
Therefore, a need exists for a detection and mitigation strategy for certain fault modes in electrical machine drive systems.
Accordingly, an object of the present invention is to provide a method and system for mitigating effects of a fault in an electrical machine drive system of an electric or hybrid electric vehicle.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.
In accordance with one aspect of the invention, a method is provided for fault mitigation in an electrical machine drive system. The electrical machine drive system includes a three-phase electrical machine, and an inverter coupled to the three-phase electrical machine. The inverter includes a first stage, a second stage, and a third stage. The first stage is coupled to a first phase of the three-phase electrical machine; the second stage is coupled to a second phase of the three-phase electrical machine; and the third stage is coupled to a third phase of the three-phase electrical machine. Each of the first stage, the second stage and the third stage includes an upper branch and a lower branch. The upper branch includes an upper power switch with an upper diode coupled across the upper power switch and the lower branch includes a lower power switch with a lower diode coupled across the lower power switch. The fault mitigation method includes the steps of: (1) opening the upper power switch and the lower power switch of each of the first stage, the second stage and the third stage of the inverter; (2) determining whether a diode or a power switch in the inverter is shorted; and (3) if a diode or a power switch in one of the first stage, the second stage, and the third stage is shorted, then commanding some power switches to close such that each phase current of the first phase, the second phase and the third phase is balanced. Ensuring balance requires insuring all lower power switches in the inverter are closed, if a lower diode or lower power switch has shorted, or ensuring all upper power switches in the inverter are closed, if an upper diode or upper power switch is shorted.
A further aspect of the present invention includes a system for fault mitigation. In addition to the electrical drive machine system described above, the system includes a control circuit and current sensors. The control circuit controls the power switches and receives inputs from the current sensors reflecting the phase currents. The control circuit implements the method steps given above.